Boring tools with cylindrical cutter chips are well known. Usually, a fluted cylindrical body which, upon rotation, defines a theoretical circle, has polygonal cutter elements, typically essentially flat, plate-like cutter elements inserted therein. The cutter elements may be essentially square, triangular, or otherwise polygonal in aspect. The cutter elements have cutting corners at the intersections of main or effective cutting edges. The cutter elements are fitted in suitable recesses in the body or shank of the boring tool. The cutter elements are radially staggered along an essentially diametrical line. A radially outer cutter having an outer cutting corner projects radially from the cutter body or shank. The main cutting edge of this cutting element may have its effective cutting edge oriented in a chord with respect to the theoretical circle formed by the cutter, that is, as the outer cutting edge rotates. A second, or inner cutter element is so positioned that its effective cutting edge intersects or comes close to the vicinity of the axis of rotation of the boring tool. The cutter elements are so located on the tool body that, in operation of the tool, and upon contact of the cutter elements with the workpiece when boring a bore hole, a radially outwardly (with respect to the axis of rotation) directed force component is generated in the tool body.
U.S Pat. No. 3,966,349, Asman describes, in general, such a boring tool. The two cutter elements have their radially outer cutting corners located on a common circle of rotation. Support strips or ridges are provided, located on the tool shank and parallel to the axis of rotation thereof, positioned adjacent the cutting corners. The cutting tool, thus, can be supported against the wall of the bore hole by these strips in combination with a radially elastically supported engagement strip. The engagement strip is positioned in the effective direction of the resulting radial component of the cutting forces. Its surface is so constructed that when it is pressed against the wall of the bore hole, in operation of the tool, it can still run on a lubricating film in the bore hole and will not destroy this lubricating film.
The support rail or support strip places the support elements engaging the cutter bits with some bias force with respect to the inner wall of the bore hole. If the workpiece is made of a material which is sensitive to scratching or, in general, substantially softer than the cutter chips, the danger may arise that as the boring tool is withdrawn from the bore which has been made, scratches or grooves may be formed in the wall of the bore hole.
The cutter insert which is positioned to intersect the axis of rotation of the boring tool has a main cutting edge which is longer than the radius of the boring tool, in other words, it extends from a radially outwardly cutting corner over and beyond the center or axis of rotation of the shank. This requires a comparatively wide cutter chip or a cutter element which is difficult to make for large bore holes, particularly when it is to be made of hard metal. Economical manufacture thereof is no longer possible. The support rails which are in engagement with the cutting edges predetermine the position of the cutter chips in the boring tool body or shank. The support rails must always be located parallel to the axis of rotation of the tool body or shank. This, then, results in comprise solutions for design of the boring tool with respect to workpiece chip formation and chip removal as well as compromise with the cutter characteristics and conditions, especially in the region of the axis of rotation of the cutting tool. In materials which are difficult to handle, and/or subject to high boring forces, and feed speeds, the compromises are difficult to achieve.